Field Coherence of EUV Sources

EUV coherence measurement and control is a challenging task which has dramatically progressed in recent years, mainly motivated by the development of sources with novel properties and their applications Non-stationary fields are an important aspect of this domain, for which numerous theoretical tools are now available. SASE XFELs have not been treated extensively in this chapter, but these devises share many common theoretical and instrumental aspects with other sources. Recently, the principle of single shot measurements of temporal and spatial coherence has been investigated using LCLS X-ray free electron laser. This technique is based on the analysis of the large angle diffraction pattern from a scattering sample, which involves path differences corresponding to greater than the expected coherence time. This idea will also be of use for other sources, in order to analyse single shot field structures or to mitigate shot to shot fluctuations. In the visible range, static Fourier-transform Michelson interferometry using echelon mirrors is already operational, and an interesting development of this technique has been proposed by Chilla, Marconi and Rocca. In this device, gratings are not only used as beam splitters but they also contribute to the generation of pulses with tilted energy fronts. This enables a complete record of the temporal coherence function in a single image and potentially in a single shot. Finally, many concepts presented here are also relevant for determining field polarisation structures, but this is beyond the scope of this chapter.